Black walnut is an important tree commercially, as the wood is a deep brown color and easily worked. The fruits, walnuts, are cultivated for their distinctive and desirable taste. Often, trees are grown for both lumber and walnuts simultaneously and many cultivars have been developed for improved quality nuts or wood. Black walnut is currently under pressure from the thousand cankers disease that is causing decline of walnuts in some areas. Black walnut is also allelopathic, which means that it releases chemicals from roots and other tissues that harm other organisms and give the tree a competitive advantage; this is often undesirable as it can harm garden plants and grasses.

Buds The buds are pale silky and covered in downy hairs. The terminal buds are ovate, and 8 mm (5⁄16 in) long, and slightly longer than broad, the lateral buds are smaller and superposed.[2]

Leaves The leaves are compound and alternately arranged on the stem. They are 30–60 cm (1–2 ft) long, typically even-pinnate but there is heavy variation among leaves. The stems have 15–23 leaflets, with the largest leaflets located in the center, 7–10 cm (23⁄4–4 in) long and 2–3 cm (3⁄4–11⁄4 in) broad.[2] The leaflets have a rounded base and a long pointed (acuminate) tip as well as having a serrated edge.[3] The leaves are overall dark green in color and are typically hairy on the underside.

Leaf scar The leaf scar has 3 prominent bundle scars and has a notch on the side that points toward the tip of the branch (distal side)

Flowers Black walnut is monoecious. The male (staminate) flowers are in drooping catkins 8–10 cm (31⁄4–4 in) long. These are borne from axilary buds on the previous year's growth. The female (pistillate) flowers are terminal, in clusters of two to five on the current year's growth.[4]

Fruit Ripens during the autumn into a fruit (nut) with a brownish-green, semifleshy husk and a brown, corrugated nut. The whole fruit, including the husk falls in October or November; the seed is relatively small and very hard.

The fruit production tends to occur irregularly with some years producing larger crops than others (see mast year). Fruiting may begin when the tree is 4–6 years old, but large crops take 20 years. Total lifespan of J. nigra is about 130 years. Black walnut does not leaf out until late spring when the soil has warmed and all frost danger is past. Like other trees of the order Fagales, such as oaks, hickories, chestnuts, and birches, it is monoecious, with wind-pollinated catkins. Male and female flowers are in separate spikes, and the female flowers typically appear before the male on a single tree (dichogamy). As a consequence, self-pollination is unlikely. However, individual trees usually are not self-sterile; if they are not pollinated by neighboring trees, they may set self-fertilized seeds.[4] For maximum seed germination, the seeds should be cold-moist stratified for 3–4 months, although the exact time depends on the seed source.[4] The seedlings emerge in April or May and typically grow 90 cm (35 in) their first year and even more in the 2nd year.[4] Black walnut often loses its leaves earlier than other deciduous trees growing in the same area after having a growing period of 115–135 days.[4]

Black walnut has a strong taproot, which makes the seedlings resilient, but difficult to transplant.

Black walnut is more resistant to frost than the English or Persian walnut, but thrives best in the warmer regions of fertile, lowland soils with high water tables, although it will also grow in drier soils, but much more slowly.[2] Some soils preferred by black walnut include Alfisol and Entisol soil types.[4] Walnut grows best on sandy loam, loam, or silt loam type soils but will also grow well on silty clay loam soils. It prefers these soils due to the fact that these soils hold large quantities of water, which the tree draws from during dry periods.[4]

Visually, black walnut is similar to the butternut (Juglans cinerea) in leaf shape, and the range also overlaps significantly. The fruits are quite different, and their presence makes an identification easy, as black walnut fruits are round and butternuts are more oval-oblong shaped. When a fruit is not available, two species can be differentiated based on the leaf scars, or the place where the leaf meets the stem: butternut has a leaf scar with a flat upper edge and with a velvety ridge above that flat part, but black walnut has an indented leaf scar with no hairy ridge.[5]

Black walnut is primarily a pioneer species similar to red and silver maple and black cherry. Because of this, black walnut is a common weed tree found along roadsides, fields, and forest edges in the eastern US. It will grow in closed forests, but is classified as shade intolerant, this means it requires full sun for optimal growth and nut production .

The nuts are food for many rodents and make up to 10% of the diet of eastern fox squirrels.[6] The nuts are also eaten by species of birds. The leaves are browsed by white tailed deer,[6] although they are not a preferred food. Squirrels benefit this species by distributing and burying the seeds; if the seeds are not reclaimed by the squirrel, they will germinate and help disperse the species.

Where the range of the eastern black walnut overlaps that of the Texas black walnut (J. microcarpa), the two species sometimes interbreed, producing populations with characteristics intermediate between the two species.[7]J.nigra and J. cinerea often grow in the same range as well but they do not hybridize naturally.[4][6]

The tree's roots often form endomycorrhizal relationships with fungi in the genus Glomus. Some endomycorrhizal relations improve the plant's growth.[4]

While its primary native region is the Midwest and east-central United States, the black walnut was introduced into Europe in 1629 and is also cultivated in Hawaii.[6] It is cultivated there and in North America as a forest tree for its high-quality wood. Black walnut plantings can be made to produce timber, nuts, or both timber and nuts. Patented timber-type trees were selected and released from Purdue University in the early 1990s. These trees have been sporadically available from nurseries. Varieties include Purdue #1, which can be used for both timber and nut production, though nut quality is poor compared to varieties selected specifically as nut producers.

Autumn foliage

Grafted, nut-producing trees are available from several nurseries operating in the U.S. Selections worth considering include Thomas, Neel #1, Thomas Myers, Pounds #2, Stoker, Surprise, Emma K, Sparrow, S127, and McGinnis. Several older varieties, such as Kwik Krop, are still in cultivation; while they make decent nuts, they would not be recommended for commercial planting. A variety index and characteristics guide is available from Missouri Extension.

Pollination requirements should be considered when planting black walnuts. As is typical of many species in Juglandaceae, Juglans nigra trees tend to be dichogamous, i.e.. produce pollen first and then pistillate flowers or else produce pistillate flowers and then pollen. An early pollen-producer should be grouped with other varieties that produce pistillate flowers so all varieties benefit from overlap. Cranz, Thomas, and Neel #1 make a good pollination trio. A similar group for more northern climates would be Sparrow, S127, and Mintle.

Sometimes black walnut is planted as part of reclaiming mines.[6] When growing young trees weed control is critical for healthy establishment of the trees, without weed control the young trees are harmed significantly in their growth rate.

Black walnut nuts are shelled commercially in the United States. About 65% of the annual wild harvest comes from the U.S. state of Missouri, and the largest processing plant is operated by Hammons Products in Stockton, Missouri. The nutmeats provide a robust, distinctive, natural flavor and crunch as a food ingredient. Popular uses include ice cream, bakery goods and confections. Consumers include black walnuts in traditional treats, such as cakes, cookies, fudge, and pies, during the fall holiday season. The nuts' nutritional profile leads to uses in other foods, such as salads, fish, pork, chicken, vegetables and pasta dishes.

Nutritionally similar to the milder-tasting English walnut, the black walnut kernel is high in unsaturated fat and protein. An analysis of nut oil from five named J. nigra cultivars (Ogden, Sparrow, Baugh, Carter and Thomas) showed the most prevalent fatty acid in J. nigra oil is linoleic acid (27.80–33.34 g/100g dry kernel), followed (in the same units) by oleic acid (14.52–24.40), linolenic acid (1.61–3.23), palmitic acid (1.61–2.15), and stearic acid (1.07–1.69).[11] The oil from the cultivar Carter had the highest mol percentage of linoleate (61.6), linolenate (5.97%), and palmitate (3.98%); the oil from the cultivar Baugh had the highest mol percentage of oleate (42.7%); the oil from the cultivar Ogden has the highest mol percentage of stearate (2.98%).

Tapped in spring, the tree yields a sweet sap that can be drunk or concentrated into syrup or sugar that is not unlike the sap of sugar maple.[citation needed]

The extraction of the kernel from the fruit of the black walnut is difficult. The thick, hard shell is tightly bound by tall ridges to a thick husk. The husk is best removed when green, as the nuts taste better if it is removed then.[citation needed] Rolling the nut underfoot on a hard surface such as a driveway is a common method; commercial huskers use a car tire rotating against a metal mesh. Some take a thick plywood board and drill a nut-sized hole in it (from one to two inches in diameter) and smash the nut through using a hammer. The nut goes through and the husk remains behind.[12]

While the flavor of the Juglans nigra kernel is prized, the difficulty in preparing it may account for the wider popularity and availability of the Persian walnut.

Black walnut drupes contain juglone (5-hydroxy-1,4-naphthoquinone), plumbagin (yellow quinone pigments), and tannin. These compounds cause walnuts to stain cars, sidewalks, porches, and patios, in addition to the hands of anyone attempting to shell them.[13]
The brownish-black dye was used by early American settlers to dye hair.[14][better source needed] According to Eastern Trees in the Petersen Guide series, black walnuts make a yellowish-brown dye, not brownish-black. The apparent confusion is easily explained by the fact that the liquid (dye) obtained from the inner husk becomes increasingly darker over time, as the outer skin darkens from light green to black. Extracts of the outer, soft part of the drupe are still used as a natural dye for handicrafts.[15]
The tannins present in walnuts act as a mordant, aiding in the dyeing process,[16][17] and are usable as a dark ink or wood stain.[18]

Walnut shells are often used as an abrasive in sand blasting or other circumstances where a medium hardness grit is required. The hard black walnut shell is also used commercially in abrasive cleaning, a filtering agent in scrubbers in smoke stacks, cleaning jet engines, cosmetics, and oil well drilling and water filtration.[6]

Black walnut is highly prized for its dark-colored, straight grained, true heartwood. It is heavy, strong, shock resistant and yet can be easily split and worked. Along with cedars (Thuja spp.), chestnut (Castanea spp.), and black locust (Robinia pseudoacacia) black walnut is one of the most durable hardwoods in the US.[6] The wood can be kiln dried and holds its shape well after seasoning, which makes this wood even more attractive for wood working.[6]

Walnut wood has historically been used for gun stocks, furniture, flooring, paddles, coffins, and a variety of other wood products.[6] Due to its value, forestry officials often are called on to track down walnut poachers; in 2004, DNA testing was used to solve one such poaching case, involving a 55-foot (16-m) tree worth US$2,500. Black walnut has a density of 660 kg per cubic meter (41.2 lb/cubic foot),[19] which makes it less dense than oak.

Maggots (larvae of Rhagoletis completa and Rhagoletis suavis) in the husk are common, though more a nuisance than a serious problem for amateurs, who may simply remove the affected husk as soon as infestation is noticed. The maggots develop entirely within the husk, thus the quality of the nutmeat is not affected.[20] However, infestations of maggots are undesirable because they make the husk difficult to remove and are unsightly. Maggots can be serious for commercial walnut growers, who tend to use chemical treatments to prevent damage to the crop.[21] Some non-chemical controls also exist, such as removing and disposing of infested nuts.[22]

The walnut weevil (Conotrachelus retentus) grows to 5 millimetres (3⁄16 in) long as an adult. The adult sucks plant juices through a snout. The eggs are laid in fruits in the spring and summer. Many nuts are lost due to damage from the larvae, which burrow through the nut shell.[23]

Black walnut is affected by European canker (Neonectria galligena). The infection spreads slowly but infected trees eventually die.[6]

The walnut caterpillar (Datana integerrima) and fall webworm (Hyphantria cunea) are two of the most serious pests, they commonly eat the foliage in midsummer and continue into autumn.

Important leaf sucking insects include species of aphids and plant lice including (Monellia spp. and Monelliopsis spp.), which suck the juices from leaves and often deposit a sticky substance called "honey-dew" on the leaf surface that may turn black and prevent photosynthesis; and the walnut lace bug (Corythucha juglandis), which causes damage when the adults and nymphs suck the sap from the lower surfaces of walnut leaflets.[4]

A disease complex known as thousand cankers disease has been threatening black walnut in several western states.[25] This disease has recently been discovered in Tennessee, and could potentially have devastating effects on the species in the eastern United States.[26] Vectored by the walnut twig beetle (Pityophthorus juglandis), Geosmithia morbida spreads into the wood around the galleries carved by the small beetles. The fungus causes cankers that inhibit the movement of nutrients in black walnut, leading to crown and branch dieback, and ultimately death.[27]

Black walnut is allelopathic, as it excretes chemicals into its environment that harm competition. While many species of plants are allelopathic, walnuts are particularly famous for it, records of walnut toxicity to other plants have been observed as far back as the first century when Pliny the Elder wrote: "The shadow of walnut trees is poison to all plants within its compass."[28] Walnuts have since been observed as being toxic to many plants including herbaceous and woody plants.

Like other walnuts, the roots, inner bark, nut husks, and leaves contain a nontoxic chemical called hydrojuglone,[28] when exposed to air or soil compounds it is oxidized into juglone that is biologically active and acts as a respiratory inhibitor to some plants. Juglone is poorly soluble in water and does not move far in the soil and will stay most concentrated in the soil directly beneath the tree.[29] Even after a tree is removed the soil where the roots once were will still contain juglone for several years after the tree is removed as more juglone will be released as the roots decay.[29] Well drained and aerated soils will host a healthy community of soil microbes and these microbes will help to break down the juglone.

Symptoms of juglone poisoning include foliar yellowing and wilting.[29] A number of plants are particularly sensitive. Apples, tomatoes, pines, and birch are poisoned by juglone, and should not be planted in proximity to a black walnut.[4][30]

The largest black walnut in Europe is located in the Castle Park in the city of Sereď, Slovakia. It has a circumference of 6.30 m (20 ft 8 in), height of 25 m (82 ft) and estimated age of 300 years.
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